1. Field of the Invention
The present invention relates to nonlinear optical chromophores and, more particularly, pertains to sterically stabilized second-order nonlinear optical chromophores and devices incorporating the same.
2. Description of the Related Art
Organic second-order nonlinear optical (NLO) materials have received increasing attention for applications involving signal processing and communications. One of the challenges in this field is to design and synthesize second-order NLO chromophores (the active components of second-order nonlinear optical materials) that simultaneously possess large first molecular hyperpolarizabilities (xcex2), good chemical and thermal stability, and optical transparency at optical communication wavelengths (1.3 and 1.55 xcexcm). Chromophore intermolecular electrostatic interactions prevent the simple scaling of molecular optical nonlinearity into macroscopic optical nonlinearity. Such interactions strongly attenuate the efficient induction of acentric chromophore order (hence, electro-optic activity) by electric field poling or self-assembly methods. Chromophores with xcex2 values many times those of the well-known Disperse Red 19 dye are thus required to obtain electro-optic coefficients comparable to or higher than those of the leading commercial material crystalline lithium niobate.
The value of xcex2 for a chromophore can be increased by using a diene moiety in place of thiophene in the conventional phenylethenylenethiophene xcfx80-conjugated bridge. Moreover, this enhancement in xcex2 can be accomplished without an increase in the wavelength of the charge-transfer absorption xcexmax. However, the resulting phenylpolyene bridge has poor thermal stability unless the polyene structure is sterically protected. The synthesis of various sterically-protected (ring-locked) phenylpolyene chromophores involves cyclic enones such as isophorone, verbenone and double-ring locked dienone as starting materials and intermediates. The Knovenegal coupling reaction between enones and electron acceptors is the critical step in both backward and forward methods reported. The low reactivity of enone severely limits the choice of acceptor to only a few molecules including malononitrile, isoxazolone, and thiobarbituric acid and therefore has become the bottleneck in the development of ring-locked phenylpolyene-bridged high xcex2 chromophores.
In addition to microscopic and macroscopic nonlinearity, the photochemical stability of second-order NLO material has long been recognized as another major problem which must be solved for successful employment of these materials in commercial devices. Chemical degradation of the NLO chromophore in polymer film can be caused by photoinduced chemical reaction and thermal decomposition. In an oxygen-containing environment (e.g., air), a major cause of chromophore degradation is photoinduced oxidation of the chromophore. Photooxidation changes the chromophore to a new species that is effectively electro-optically inactive.
Researchers have been trying to improve the photochemical stability of NLO chromophores by modifying their chemical structure. The results of their efforts indicate that the stability can be improved by avoiding adjacent alkyl groups on the nitrogen donor of the chromophore [See, U.S. Pat. No. 5,776,375 to Hofstraat, et al.], by using aromatic substituents on the nitrogen donor, by using azo bridge linkage instead of a carbon-carbon double bond, and by attaching a radical scavenger to the chromophore [See, Optics Letters, 2000, Vol. 25, no. 5, 332-334, Andriana Galvan-Gonzalz, et al.]. It has also been found that shorter chromophores generally have higher photochemical stability in air. However, due to the strong dependence of molecular nonlinearity on chromophore conjugation length, short chromophores cannot provide a sufficient degree of nonlinearity for practical applications [Cheng, L.-T.; Tam, W.; Marder, S. R.; Stiegman, A. E.; Rikken, G.; Spangler, C. W. J. Phys. Chem. 1991, 95, 10643-10652. Marder, S. R.; Cheng, L.-P.; Tiemann, B. G.; Friedli, A. C.; Blanchard-Desce, M.; Perry, J. W.; Skindhxc3x8j, J. Science, 1994, 263, 511-514. Wong, K. Y.; Jen, A. K.-Y.; Rao, V. P. Phys. Rev. 1994, 49, 3077-3080.].
In addition to the chemical decomposition of the chromophore, light can also cause randomization of electric poling-induced alignment. In this process, although the chromophore chemical structure is not destroyed, the material will still lose all nonlinear activity evantually. To address this aspect of the photo-related problem, one would need to modified the chromophore structure to reduce or eliminate certain structural units that could lead to the unwanted random motion of the chromophore backbone, and to find a more rigid yet processible polymer host to restrict free motion of the chromophore. Since the properties (microscopic nonlinearity, macroscopic, chemical and thermal stability, etc.) of second-order nonlinear optical materials are inter-related, optimization of one property often causes unacceptable amounts of attenuation in other properties. Thus, a systematic approach to addressing both the stability and nonlinearity issues is needed for a balanced improvement in both properties. An approach based on chromophore structure modification has been addressed in the parent U.S. patent application Ser. No. 09/546,930. The present invention provides a solution to the photochemical issue associated with NLO materials, a solution which does not sacrifice requirements of molecular nonlinearity and high poling efficiency.
A new class of ring-locked aminophenylpolyenal donor-bridges has been developed. These new donor-bridges, according to the present invention, have very high Knovenegal reactivity and have been coupled with a variety of acceptors bearing acidic methyl or methylene groups (including the most desired TCF and TCI type of acceptors shown in FIG. 11) to obtain a new class of second-order NLO chromophores. This methodology broadens the scope of polyene-bridged chromophores without significantly sacrificing thermal stability or optical transparency. This synthetic approach leads to the development of device-quality NLO chromophores (shown in FIG. 1) possessing xcexcxcex2 values (where xcexc is the chromophore dipole moment) of 15,000xc3x9710xe2x88x9248 esu or greater at 1.9 xcexcm as determined by the electric field induced second harmonic generation (EFISH) technique.
A variety of different molecular structures are possible for the chromophores of the present invention. An exemplary preferred basic chromophore structure according to the present invention includes an electron donor group, an electron acceptor group and a xcfx80-conjugate bridge structure therebetween. The bridge is a polyene structure having a five-, six- or seven-membered ring to lock one carbon-carbon double bond. Uniquely, the bridge contains an unlocked conjugate diene unit to connect the bridge ring and the acceptor (A). In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an electron acceptor group and a ring-locked bridge structure between the electron donor group and the electron acceptor group. The bridge structure comprises a fused double- or triple-ring structure which functions to lock two or three double bonds. The bridge also contains an unlocked conjugate diene unit to connect the bridge ring and the acceptor (A). In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an acceptor, and a bridge structure therebetween, wherein the acceptor is a five- or six-membered ring-locked tricyano electron acceptor.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an electron acceptor group, and a bridge structure therebetween, with the bridge structure including a bithiophene unit. In a preferred embodiment, the bithiophene unit is modified with side group(s).
The NLO materials of the present invention are suitable for a wide range of devices. Functions performed by these devices include, but are not limited to, the following: electrical to optical signal transduction; radio wave to millimeter wave electromagnetic radiation (signal) detection; radio wave to millimeter wave signal generation (broadcasting); optical and millimeter wave beam steering; and signal processing such as analog to digital conversion, ultrafast switching of signals at nodes of optical networks, and highly precise phase control of optical and millimeter wave signals. These materials are suitable for arrays which can be used for optical controlled phased array radars and large steerable antenna systems as well as for electro-optical oscillators which can be used at high frequencies with high spectral purity.
A new approach according to the present invention for solving the critical photochemical stability problem has been developed by studying the photochemical decomposition behavior of high xcexcxcex2 chromophores in air and in an inert gas environment. More specifically, it has been observed that the careful removal of oxygen from the material in the device and from the environment of the device yields a material which is stable at high optical power for 5 days without any observable drop in its optical properties. This result demonstrates that the major mechanism of chromophore photodegradation, the photochemical degradation of chromophores, can be eliminated by removing oxygen from the material in the device and from the environment of the device. According to the present invention, any polymeric electro-optic device (especially those using high xcexcxcex2 chromophores) can be protected from oxygen thus solving the problem of chromophore photodegradation. An exemplary preferred way of providing this protection according to the present invention is to hermetically package the device in a container filled with an inert gas. Another exemplary preferred way of providing this protection according to the present invention is to insulate the device from air (oxygen, in particular) by coating the electro-optic device with a polymeric material which has a very low permeativity for oxygen.